Epstein-Barr virus (EBV) has been detected in lymphoepithelioma of nasopharynx and lymphoepitheliomalike carcinomas in various organs. To clarify the association of EBV with gastric carcinoma with lymphoid stroma, which often resembles lymphoepithelioma, the authors examined 22 such cases by polymerase chain reaction and in situ hybridization techniques. In 18 informative cases, EBV DNA was detected by polymerase chain reaction in 14 (77.8%) cases, including lymph node metastases. EBV RNA was detected within the nuclei of carcinoma cells by in situ hybridization in all cases that were positive by polymerase chain reaction. Infiltrating lymphocytes and normal epithelia adjacent to carcinoma were EBV-negative. Southern blot analysis indicated clonal proliferation of tumor cells and episomal form of EBV. These findings suggest that EBV infection occurs before transformation and may be related to oncogenesis of EBV-associated gastric carcinoma.